The present invention relates to the general field of aviation.
More particularly, it relates to monitoring a servo-control loop of an actuator system for actuating variable-geometry components of a turbojet.
The term “variable-geometry components” is used herein to mean components of equipment other than the rotary members of the engine and that present dimensions, shapes, and/or positions and/or speeds that can be modified as a function of detected events or as a function of engine speeds. Examples of “variable-geometry components” include pitch angles of variable-pitch stator vanes (VSVs) of compressors, variable bleed valves (VBVs) of the compressor, turbine blade tip clearances, and the positions of a fuel metering unit.
In traditional manner, these variable dimensions, shapes, or positions are varied by hydraulic actuator systems that use fuel as their actuating fluid. Such actuator systems are controlled by the electronic control unit (ECU) of the engine, so as to adapt the variable-geometry components to flight conditions. Control is performed via servo-control loops.
Thus, it can readily be understood that it is crucial to monitor the actuator systems of variable-geometry components in order to ensure that a turbojet is available and efficient.
Degradations to such actuator systems lead, in particular, to components of the turbojet taking up positions and/or sizes that do not comply with the controls from the ECU under steady conditions, or to said components responding slowly to such controls under transient conditions. Such degradations are early signs of failure since in general they are initially either compensated by the servo-control loops of the actuator systems or else without any significant consequence other than a reconfiguration (e.g. changing the active control channel).
Nevertheless, after a certain amount of time, when such degradations persist and get worse, they are no longer compensated and they can have the consequence of making a turbojet inoperative or of reducing its performance. That can lead to the ECU issuing a failure message. Such deteriorations are thus detected too late when they are detected only as a result of the actuator system breaking down.
By way of example, document FR 2 939 924 describes a technique of detecting failures in an aeroengine, the technique involving estimating a plurality of indicators.
There thus exists a need to have a method for effectively monitoring the servo-control loops of the actuator systems for actuating variable-geometry components of a turbojet, in particular in order to be able to issue a maintenance notice for such actuator systems before the turbojet is made inoperative or inefficient, while taking care to minimize the number of useless or untimely maintenance operations that are performed.
The document “Hydraulic actuation loop degradation diagnosis and prognosis”, by E. Dize-Lledó, J. Aguila-Margin, J-R. Massé, A. Sif, and E. Griful, 1st CEAS European Air and Space conference, 2007, describes the use of diagnosis and prognosis techniques applied to monitoring servo-control loops of systems for actuating variable-pitch stator vanes.